Free cutting steel

ABSTRACT

A free cutting steel for construction of machines, characterized in that the deoxidation product remaining in the steel has a composition of 40-60% of SiO 2 , 10-30% of MnO and 5-25% of Al 2  O 3 , the balance being other oxides such as CaO and the sum of contents of SiO 2 , MnO and Al 2  O 3  being so adjusted as to be at least 85% of the whole oxides in the deoxidation products, the steel being further incorporated with 0.02 - 0.10% of Pb, and that the non-metallic composite inclusions comprising sulfides and finely divided Pb particles are uniformly dispersed.

This application is a continuation-in-part application of applicationSer. No. 275,900, filed July 28, 1972, now abandoned.

This invention relates to free cutting steels. More particularly, theinvention relates to free cutting steels comprising metal components ofsteels referred to as carbon steel, manganese steel, manganese-chromiumsteel and alloy steel for the machine structural use in which themachinability has been improved by allowing the presence of suitableamounts of oxide inclusion of SiO₂ -MnO-Al₂ O₃ system and compositeinclusions comprising sulfides and finely divided lead particlesuniformly dispersed in the steel.

Various kinds of steels useful for manufacturing machine parts are wellknown. They include plain carbon steels and alloy steels. Recently, freecutting steels have been used broadly because it has been found thatthese steels can be cut at high speed with cemented carbide tools, andthe tool life can be improved. However, in case free cutting steels ofthe Ca-type are cut with high speed steel tools, the improvement in thetool life is not prominent.

It is an object of the present invention to provide a free cutting steelwhich shows an improved tool life in cutting with tools, especially highspeed steel tools.

It is another object of the present invention to provide a free cuttingsteel which possesses not only an excellent machinability but also goodmechanical properties.

This invention will now be illustrated detailedly by reference toaccompanying drawings.

FIG. 1 is an enlarged photograph illustrating the state of non-metallicinclusions present in the conventional free cutting steel of the Ca-Pbtype;

FIG. 2 is an enlarged photograph illustrating the state of non-metallicinclusions present in the free cutting steel of the present invention;

FIG. 3 is a triangular coordinate of the phase diagram for the systemSiO₂ -CaO-Al₂ O₃, of the deoxidation product in the conventional freecutting steel of the Ca-Pb type;

FIG. 4 is a triangular coordinate of the phase diagram for system SiO₂-MnO-Al₂ O₃, of the deoxidation product in the free cutting steel of thepresent invention;

FIGS. 5 and 6 are graphs illustrating the life of a cemented carbidetool at the high speed cutting test of SCM 22-type steel and S48C-typesteel;

FIG. 7 is a view illustrating the critical strain in cold formabilitytest of SCM 22-type steel;

FIG. 8 is a graph illustrating rolling contact fatigue life of S48C-typesteel.

The inventors have made a many experiments on morphology of non-metallicinclusion in Ca-Pb type free cutting steel, and found that Pb formscomposites of a large size with oxides and therefore, the dispersion ofPb in the steel is insufficient in such case. It was also found that thecomposition of oxide inclusions of almost all of these free cuttingsteels is approximately 40% Al₂ O₃, 40% SiO₂ and 20% CaO. As a result ofthe investigation of the micro-structures of the Ca-Pb type free cuttingsteel containing a deoxidation product comprising such oxide inclusionsas main ingredients, it was found that non-metallic inclusions consistmainly of composite inclusions formed from oxide inclusions and Pbparticles, as shown in the photograph of FIG. 1-a, and inclusionscomposed solely of sulfide, as shown in the photograph of FIG. 1-b.

With a view to providing improved free cutting steels withoutabove-mentioned defects, the inventors have done considerable researchand found that satisfactory results can be obtained when compositeinclusions composed of sulfides and finely divided lead particles areformed in the steel, and further, that the composite inclusions will beformed by reducing the melting point of the deoxidation product.Experiments for manufacturing the free cutting steel according thepresent invention were made for instance by the following procedures:

Molten steel was obtained by a conventional steel-making process in anelectric arc furnace. Slag floating over the molten steel was removedand the oxygen content of the steel was adjusted to be in the range of80 to 300 ppm. Then, the molten steel was tapped into a ladle and addedwith a low aluminum ferrosilicon deoxidizing alloy (composition: 18% Si,0.1% Al, 0.01% C and the balance iron) in an amount of about 3 Kg/tonsteel. At the tapping, lead was added to the molten steel stream at arate of about 8 Kg/ton steel. As a result, deoxidation products floatedup over the molten steel, and oxide inclusion of about 100 to 450 gramsper ton steel remained as uniform dispersion

in the steel with finely divided lead particles.

The composition of the oxide inclusion was: SiO₄ 40 to 60%, MnO 10-30%,Al₂ O₃ 5 to 25% and MO up to 15%. The steel also contained the compositeinclusions composed of sulfides and finely divided lead particles.

It has been confirmed that non-metallic inclusions of the so formed freecutting steel consists of inclusions comprised oxides as shown in FIG.2-a and composite inclusions composed of sulfides divided Pb as shown inFIG. 2-b. It was also confirmed that the dispersion of such inclusionsis very desirable one. Further, it was found that the cutting propertyof the free cutting steel in which the composite inclusions areuniformly dispersed, is extremely improved over the conventional freecutting steel of the Ca-Pb type, and the mechanical strength of suchfree cutting steel is quite comparable to that of the conventional freecutting steel of the Ca-Pb type.

Thus, in accordance with this invention, there is provided a freecutting steel for construction of machines, characterized in that thedeoxidation product remains in the steel as an oxide inclusion in anamount within a range from 100 to 450 grams per ton of the steel and hasa composition of 40-60% of SiO₂, 10-30% of MnO and 5-25% of Al₂ O₃, thebalance being other oxides such as CaO and the sum of contents of SiO₂,MnO and Al₂ O₃ being so adjusted as to be at least 85% of the wholeoxides in the deoxidation products, the steel being further incorporatedwith 0.02-0.10% of Pb, and that the non-metallic composite inclusionscomprising sulfides and finely divided Pb particles are uniformlydispersed.

The components of the deoxidation product of the free cutting steel ofthe present invention will now be described. In order to obtain oxideinclusions which can readily be deformed by not working and can beelongated slenderly along the working and rolling direction, it isdesirable to adjust the content of SiO₂ to at least 40%. However, incase SiO₂ is present together with 10-30% of MnO and 5-25% of Al₂ O₃,the content of SiO₂ exceeding 60% is not preferred for purpose ofreduction of the melting point of the deoxidation product. In order tomaintain lower the melting point of the deoxidation product, it ispreferred that MnO is present in a content of at least 10%. However, inatmospheric melting and smelting, the MnO content which is presenttogether with 40-60% of SiO₂ and 5-25% of Al₂ O₃, is quite difficult toincrease above 30%. Therefore, the MnO content is adjusted within therange of 10 to 30 %. In order to obtain a deoxidation product havinglower melting point, it is preferred that the content of Al₂ O₃ islower. However, in atmospheric melting and smelting, it is difficult toreduce the Al₂ O₃ content below 5%, if Al₂ O₃ is present together with40-60% of SiO₂ and 10-30% of MnO. Too high content of Al₂ O₃ results inincrease of the softening and melting point of the deoxidation product.Therefore, it is not preferred to increase the content of the Al₂ O₃above 25%.

It is usually inevitable that other oxides such as CaO, FeO and MgO areincorporated in the deoxidation product. It is desirable to adjust thecontent of such oxide impurities to a level not interferring with theabove object, that is, within a range of up to 15%. It has been foundthat in order to reduce the melting point of the deoxidation productcomposed of SiO₂, MnO, Al₂ O₃ and other oxides such as CaO, the sum ofthe contents of SiO₂, MnO and Al₂ O₃ must be preferably at least 85% asa result of experiments.

It is necessary to adjust the content of the oxide inclusion to fallwithin a range from 100 to 450 grams per ton of the steel in order toobtain a desirable uniform distribution of the finely divided leadparticles and sulfides in the steel.

The present invention is applicable to most carbon steels, manganesesteels and manganese-chromium steel for machine structural use and otherstructural alloy steels; i.e., steels of many grades can be employed forpreparing the free-cutting steel of the present invention.Representatives of such steels and their composition ranges are shown inTable 1.

                  Table 1                                                         ______________________________________                                        Steel Mark   Composition (%)                                                             C    Si     Mn      Ni   Cr    Mo                                  ______________________________________                                        Carbon Steel                                                                  SAE 1010-1055                                                                              0.08   0.10   0.30  --   --    --                                             |                                                                           |                                                                           |                                                      0.60   0.30   1.65                                               JIS G 4051   0.08   0.15   0.30  --   --    --                                             |                                                                           |                                                                           |                                                      0.61   0.35   0.90  --   --    --                                S48C, S45C   0.40   0.15   0.60  --   --    --                                SAE 143,1045,                                                                              |                                                                           |                                                                           |                                         1046,1049,1050                                                                             0.55   0.35   1.00  --   --    --                                Chromium Steel                                                                SAE 5015,5040,5060                                                                         0.12   0.20   0.30  --   0.30  --                                5115-5160    |                                                                           |                                                                           | |                                           0.64   0.35   1.00       1.15                                    JIS G 4104   0.13   0.15   0.60       0.90                                                 |                                                                           |                                                                           |                                         SCr 2-5,21,22                                                                              0.48   0.35   0.85  --         --                                Chromium-Molybdenum Steel                                                     JIS G4105     0.18  0.15   0.30       0.70  0.15                                           |                                                                           |                                                                           | |                                                                          |                        SCM 1-5,21-24                                                                              0.65   0.35   1.00  --   1.20  0.35                              SAE 4118-4161                                                                 JIS G 4105   0.18   0.15   0.60       0.90  0.15                                           |                                                                           |                                                                           | |                                                                          |                        SCM 22       0.23   0.35   0.85       1.20  0.30                              Manganese Steel                                                               JIS G 4106   0.17   0.15   1.20  --   --    --                                SAE 1330-1345                                                                              |                                                                           |                                                                           |                                         SMN 1-3,21   0.48   0.35   1.90                                               Molybdenum Steel                                                              SAE 4012-4047,                                                                             0.09   0.20   0.65  --   --    0.15                              4419-4427    |                                                                           |                                                                           |       |                                     0.50   0.35   1.00             0.60                              Manganese-Chromium Steel                                                      JIS G4106    0.17   0.15   1.20  --   0.35  --                                             |                                                                           |                                                                           | |                              SMN C3,21    0.46   0.35   1.65       0.70                                    Nickel-Chromium Steel                                                         JIS G4102    0.12   0.15   0.35  1.00 0.20                                                 |                                                                           |                                                                           |                                                                          |                                                                         |                                                                          --                                SNC 1-3,21,22                                                                              0.40   0.35   0.80  3.50 1.00                                    Nickel-Chromium-Molybdenum Steel                                              SAE 4320,4340                                                                 4718,4720,8115                                                                             0.08   0.20   0.45  0.40 0.30  0.08                              8615-8660,8720,                                                                            |                                                                           |                                                                           |                                                                          |                                                                         |                                                                          |                        8740 8822,9260,9310                                                                        0.64   0.35   1.00  3.50 1.40  0.40                              JIS G4103    0.12   0.15   0.35  0.40 1.40  0.15                              SNCM 1-9,21-26                                                                             |                                                                           |                                                                           |                                                                          |                                                                         |                                                                          |                                     0.50   0.35   1.20  4.50 3.50  0.70                              ______________________________________                                         *The balance is iron and inherent impurities.                            

This invention will now be more illustrated in detail by reference toExamples.

EXAMPLE 1

Steels containing deoxidation producrs of the composition indicated inTable 2 were prepared, and the tool life was determined with respect toeach of these steels. Results are shown in Table 3.

                                      Table 2                                     __________________________________________________________________________                         Chemical Composition (%)                                 No.                                                                              Samples   Steel Mark                                                                            C   Si   Mn   P    S    Cr  Mo  Pb   Ca                  __________________________________________________________________________    a-1                                                                              Conventional                                                                            SCM 22-C1                                                                             0.20                                                                              0.27 0.65 0.015                                                                              0.020                                                                              1.01                                                                              0.18                                                                              0.18 0.003                  steel                                                                      a-2                                                                              Conventional                                                                            SCM 22-C2                                                                             0.21                                                                              0.29 0.67 0.018                                                                              0.021                                                                              1.00                                                                              0.19                                                                              0.19 0.004                  Steel                                                                      A-1                                                                              Steel of This                                                                           SCM 22-A1                                                                             0.21                                                                              0.24 0.69 0.016                                                                              0.036                                                                              1.01                                                                              0.19                                                                              0.06 --                     Invention                                                                  A-2                                                                              Steel of This                                                                           SCM 22-A2                                                                             0.21                                                                              0.25 0.72 0.015                                                                              0.016                                                                              1.03                                                                              0.16                                                                              0.10 --                     Invention                                                                  A-3                                                                              Steel of This                                                                           SCM 22-A3                                                                             0.18                                                                              0.24 0.69 0.019                                                                              0.016                                                                              1.03                                                                              0.20                                                                              0.02 --                     Invention                                                                  a-3                                                                              Conventional                                                                            S45C-C  0.44                                                                              0.25 0.76 0.016                                                                              0.017                                                                              --  --  0.16 0.002                  Steel                                                                      A-4                                                                              Steel of This                                                                           S45C-A  0.45                                                                              0.24 0.69 0.015                                                                              0.018                                                                              --  --  0.08 --                     Invention                                                                  __________________________________________________________________________                   Composition of Deoxidation Product (%)                                                                           Melting Point                                                                 (°C.)                                                                  Synthetic Slag having                                                  (SiO.sub.2 +MnO)                                                                     same composition as         No.                                                                              Samples     SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   others                                                                            (SiO.sub.2 +MnO+Al.sub.2 O.sub.3)                                                        (Al.sub.2 O.sub.3)                                                                   deoxidation                                                                   product.sup.1)              __________________________________________________________________________    a-1                                                                              Conventional Steel                                                                        28   5  39   1   72         0.85   1,450                       a-2                                                                              Conventional Steel                                                                        30   5  45   1   80         0.78   1,470                       A-1                                                                              Steel of This Inven-                                                                      43  22  21   2   86         3.1    1,270                          tion                                                                       A-2                                                                              Steel of This In-                                                                         51  25  17   2   93         4.5    1,210                          vention                                                                    A-3                                                                              Steel of This In-                                                                         46  27  18   1   91         4.1    1,230                          vention                                                                    a-3                                                                              Conventional Steel                                                                        36  trace                                                                             42   1   99         0.9    1,520                       A-4                                                                              Steel of This In-                                                                         45  16  25   1   86         2.4    1,360                          vention                                                                    __________________________________________________________________________     .sup.1) the melting point was determined by means of Seger Cone.         

                                      Table 3                                     __________________________________________________________________________                   Life of      0.2%      Reduc-                                                                             Charpy                                            high speed                                                                            Tensile                                                                            proof                                                                              Elonga-                                                                            tion of                                                                            impact                                            steel tool                                                                            strength                                                                           stress                                                                             tion area value                              No. Samples    (V=80m/min)                                                                           (kg/mm.sup.2)                                                                      (kg/mm.sup.2)                                                                      (%)  (%)  (kg.m/cm.sup.2)                                                                     Heat Treatment               __________________________________________________________________________    a-1 Conventional Steel                                                                       75       98.5                                                                              70.0 20.5 58.3 12.5                               a-2 Conventional Steel                                                                       82      100.3                                                                              68.9 20.3 57.9 12.3  870°C×30minu                                                     tes and                                                                       oil quenching;               A-1 Steel of this In-                                                                        220     110.3                                                                              71.3 20.8 58.5 13.4  830°C×30minu                                                     tes and                          vention                                      oil quenching;               A-2 Steel of this In-                                                                        250     105.2                                                                              73.1 19.5 58.4 12.7  200°C×60minu                                                     tes and                          vention                                      water tempering              A-3 Steel of this In-                                                                        240      99.8                                                                              70.5 19.7 58.5 12.4                                   vention                                                                   a-3 Conventional Steel                                                                       20      125.3                                                                              80.3 18.5 50.1  7.5  840°C and air                                                          cooling                      A-4 Steel of this In-                                                                        35      124.1                                                                              79.8 18.0 51.0  8.0  835°C and water                                                        quenching                        vention                                      600°C and oil                                                          tempering                    __________________________________________________________________________

We show the phase diagram for the system of SiO₂ -CaO-Al₂ O₃ of thedeoxidation product of conventional free cutting steels of the Ca-Pbtype in FIG. 3. From the FIG. 3 and the Table 2, it is observed that themelting points of the deoxidation product of the above conventional freecutting steels are above 1,450°C. According to the steel of the presentinvention, the composition of the oxide inclusions is maintained withinthe above-mentioned specific range, and the relation of these threeoxides are shown in the phase diagram for the system of SiO₂ -MnO-Al₂ O₃of FIG. 4, and it was found that the melting point of the deoxidationproduct is below 1,350°C.

Namely, the steel of the present invention shows an extremely improvedtool life as shown in Table 3 on account of reduction of the meltingpoint of the deoxidation product and uniform distribution of finelydivided Pb particles and sulfides in the form of non-metallic inclusionssuch as illustrated in FIG. 2.

Further, it was also found that, as shown in Table 3, the steel of thisinvention is a free cutting steel of high quality of which themechanical properties are superior or comparable to the conventionalfree cutting steel of the Ca-Pb type.

We prepared steels having compositions shown in Table 4, and testspecimens thereof were subjected to the high speed cutting test underthe turning conditions indicated in Table 5. Results are shown in FIGS.5 and 6. Namely, it was confirmed that the improvement in the tool lifeattained by the steel of the present invention is superior or comparableto that attained in the conventional free cutting steels of the Ca-Pb-Stype. Thus according to the steels of this invention, the tool life atlow speed cutting by a high speed steel tool (SKH 4) (see Table 3) andat high speed cutting by a cemented carbide tool has been extremelyimproved. As is shown in Table 6, further, the cutting scrap obtained bycutting of the steel of the present invention with high speed steel toolcould be very easily crushed during cutting.

                                      Table 4                                     __________________________________________________________________________                   Chemical Composition, %                                        No.                                                                              Samples                                                                            Steel Mark                                                                           C  Si Mn P   S   Ni   Cr Mo Pb Ca                              __________________________________________________________________________    b-1     SCM22-C11                                                                            0.21                                                                             0.20                                                                             0.77                                                                             0.010                                                                             0.020    1.12                                                                             0.18                                                                             -- --                              b-2                                                                              Conven-                                                                            SCM22-C12                                                                            0.21                                                                             0.26                                                                             0.75                                                                             0.012                                                                             0.020                                                                             --   1.10                                                                             0.18                                                                             0.18                                                                             --                              b-3                                                                              tional                                                                             SCM22-C13                                                                            0.20                                                                             0.25                                                                             0.75                                                                             0.015                                                                             0.015                                                                             Nb 0.04                                                                            0.99                                                                             0.18                                                                             -- 0.002                           b-4                                                                              steels                                                                             SCM22-C14                                                                            0.21                                                                             0.27                                                                             0.73                                                                             0.013                                                                             0.055                                                                             Nb 0.05                                                                            1.09                                                                             0.19                                                                             -- 0.003                           b-5     SCM22-C15                                                                            0.21                                                                             0.29                                                                             0.72                                                                             0.016 0.048                                                                       Nb 0.05                                                                           1.04 0.20                                                                             0.14                                                                             0.001                                 Steel of                                                                   B-1                                                                              this in-                                                                           SCM22-A11                                                                            0.20                                                                             0.26                                                                             0.75                                                                             0.015                                                                             0.040                                                                             Nb 0.03                                                                            1.00                                                                             0.20                                                                             0.06                                                                             --                                 vention                                                                    b-6     S48C-C1                                                                              0.47                                                                             0.26                                                                             0.74                                                                             0.017                                                                             0.025                                                                             --   -- -- -- --                              b-7                                                                              Conven-                                                                            S48C-C2                                                                              0.52                                                                             0.26                                                                             0.72                                                                             0.015                                                                             0.020                                                                             --   -- -- 0.17                                                                             --                              b-8                                                                              tional                                                                             S48C-C3                                                                              0.47                                                                             0.27                                                                             0.76                                                                             0.014                                                                             0.015                                                                             --   -- -- -- 0.002                           b-9                                                                              Steels                                                                             S48C-C4                                                                              0.49                                                                             0.29                                                                             0.75                                                                             0.016                                                                             0.062                                                                             --   -- -- 0.22                                                                             0.001                              Steel of                                                                   B-2                                                                              this in-                                                                           S48C-A1                                                                              0.46                                                                             0.31                                                                             0.72                                                                             0.016                                                                             0.028                                                                             --   -- -- 0.06                                                                             --                                 vention                                                                    __________________________________________________________________________    Deoxidation  Composition of deoxida-                                          Product      tion product, %         (SiO.sub.2)+(MnO)+(Al.sub.2 O.sub.3)     No. (g/ton steel)                                                                          SiO.sub.2                                                                           MnO   Al.sub.2 O.sub.3                                                                    others                                                                              (%)                                      __________________________________________________________________________    b-1 135       4    --    95     1    99                                       b-2 130       3    --    93     4    96                                       b-3 215      33    8     39    20    80                                       b-4 233      22    6     54    18    82                                       b-5 218      28    10    41    21    79                                       B-1 225      48    25    19     8    92                                       b-6  85       2    --    97     1    99                                       b-7  76       2    --    95     3    97                                       b-8 124      26    1     35    38    62                                       b-9 121      30    7     45    18    82                                       B-2 138      46    17    25    12    88                                       __________________________________________________________________________

                  Table 5                                                         ______________________________________                                        Cutting Tool P10 (cemented carbide tool consist-                                           ing of 63% of WC, 38% of (TiC = TaC)                                          and 9% of Co)                                                    Tool Form    -5, -5, 5, 5, 30, 0, 0, 0.4                                      Cutting Speed                                                                              0.20 mm/rev.                                                     Tool Life    V.sub.B = 0.3 mm (determined based on the                                     time required for flank abrasion of                                           the tool to reach 0.3 mm)                                        Heat Treatment of                                                                          normalizing at 900°C for 2 hours and                      Steels       air cooling (steel of SCM 22-type),                                           or normalizing at 850°C for 2 hours                                    and air cooling (steel of S48C-type)                             Hardness     HRB = 165-170 (steel of SCM 22-type),                                         or                                                                            HRB = 201-210 (steel of S48C-type)                               Cutting Oil  None                                                             ______________________________________                                    

                  Table 6                                                         ______________________________________                                        Breakability of Cutting Chip                                                  No.       Steels          Breakability                                        ______________________________________                                        b-1       SCM22-C11       X                                                   b-2       SCM22-C12       0                                                   b-3       SCM22-C13       X                                                   b-4       SCM22-C14       Δ                                             b-5       SCM22-C15       0                                                   B-1       SCM22-A11       0                                                   b-6       S48C-C1         X                                                   b-7       S48C-C2         0                                                   b-8       S48C-C3         X                                                   b-9       S48C-C4         0                                                   B-2       S48C-A1         0                                                   Remarks:                                                                      (1)   Cutting Conditions                                                            Tool            SKH4 0, α, 7, 7, 10, 0, 0                                                  (α = 0°, 8°, 16°)               Cutting speed   40, 60, 80 m/min.                                             Feed            0.05, 0.10, 0.15 mm/rev.                                      Depth of Cut    0.15, 0.30, 0.45, 0.60, 0.90,                                                 1.20, 180 mm.                                                 Repeated 27 tims                                                        (2)   Evaluation:                                                             X : no good    Δ : fairly good                                                                        0 : good.                                       ______________________________________                                    

In order to examine the cold workability of the SCM 22-type steels ofthe present invention, the critical strain of test specimens (diameter =8 mm; length = 16 mm) as normalized (N ) and as spheroidizing annealed(SA) were determined under the upset condition. As the results shown inFIG. 7, it was confirmed that the workability of the steel according tothe present invention in cold working is almost to that of theconventional steels (SCM 22-C11 or SCM 22-C13).

FIG. 8 illustrates results of determination of the rolling contactfatigue life of the S48C-type steel of the present invention, which hadbeen heat-treated so that the steel had a Rockwell hardness C of 63.From the FIG. 8, it is seen that the rolling contact fatigue life of thesteel of this invention is not substantially different from that of thebasic steel (S48C1).

EXAMPLE 2

Steels of the composition indicated in Table 7 were prepared.

The steel specimens were tempered and subjected to cutting test underthe conditions indicated in Table 8.

The results as high speed steel tool life for each specimens are shownin Table 9.

                                      Table 7                                     __________________________________________________________________________                                       Composition of Deoxi-                                                 Deoxidation                                                                           dation Product (%)                         No. Samples                                                                              Chemical Composition (%)                                                                      Product                (SiO.sub.2)+(MnO)+(Al.su                                                      b.2 O.sub.3)                           C  Si Mn Pb Ca  (g/ton steel)                                                                         SiO.sub.2                                                                        MnO                                                                              Al.sub.2 O.sub.3                                                                   Others                                                                            (%)                         __________________________________________________________________________    C-1        0.08                                                                             0.15                                                                             0.50                                                                             0.05                                                                             --  450     51 25  9   15  85                              Steel of                                                                  C-2        0.20                                                                             0.11                                                                             0.35                                                                             0.02                                                                             --  328     57 22 10   11  89                              this Inven-                                                               C-3        0.31                                                                             0.21                                                                             1.45                                                                             0.08                                                                             --  253     50 17 21   12  88                              tion                                                                      C-4        0.40                                                                             0.25                                                                             1.62                                                                             0.06                                                                             --  170     43 13 24   15  85                          C-5        0.50                                                                             0.35                                                                             0.90                                                                             0.04                                                                             --  125     49 12 24   15  85                          C-6        0.61                                                                             0.30                                                                             0.79                                                                             0.07                                                                             --  103     50 10 25   11  85                          __________________________________________________________________________    c-1        0.09                                                                             0.14                                                                             0.48                                                                             -- 0.003                                                                             403     60 1  21   18  82                          c-2        0.19                                                                             0.12                                                                             0.35                                                                             -- 0.002                                                                             307     48 0  39   13  87                              Conventional                                                              c-3        0.30                                                                             0.22                                                                             1.42                                                                             -- 0.001                                                                             215     26 1  55   18  82                              Steels                                                                    c-4        0.42                                                                             0.23                                                                             1.65                                                                             -- 0.003                                                                             125     31 0  59   10  90                          c-5        0.49                                                                             0.34                                                                             0.87                                                                             -- 0.002                                                                             115     23 0  68   14  86                          c-6        0.58                                                                             0.28                                                                             0.82                                                                             -- 0.004                                                                              92     21 0  63   16  84                          __________________________________________________________________________

                  Table 8                                                         ______________________________________                                        Cutting Conditions                                                            Tool              SKH 57 (0, 15, 7, 7, 10, 0,                                                       0.5)                                                    Feed              0.12 mm/rev.                                                Cutting Speed     80 m/min                                                    Depth of Cut      1 mm                                                        Cutting Oil       Water insoluble                                             Tool life is determined by melting thereof.                                   ______________________________________                                    

                  Table 9                                                         ______________________________________                                                                Tool Life of High Speed                               No.      Samples        Steel Tool (min)                                      ______________________________________                                        C-1                     950                                                   C-2                     420                                                            Steel of                                                             C-3                     85                                                             this In-                                                             C-4                     33                                                             vention                                                              C-5                     30                                                    C-6                     12                                                    ______________________________________                                        c-1                     480                                                   c-2                     230                                                            Conventional                                                         c-3                     35                                                             Steels                                                               c-4                     15                                                    c-5                     20                                                    c-6                     7                                                     ______________________________________                                    

EXAMPLE 3

Steels having the compositions indicated in Table 10 were prepared.

The tempered steel specimens were subjected to cutting test under theconditions indicated also in Table 8.

Table 11 shows the high speed steel tool life.

                  Table 11                                                        ______________________________________                                                                Tool Life of High Speed                               No.      Samples        Steel Tool (min)                                      ______________________________________                                        D-1                     280                                                   D-2      Steel of       130                                                   D-3      this In-       68                                                    D-4      vention        41                                                    D-5                     18                                                    ______________________________________                                        d-1                     150                                                   d-2      Conventional   65                                                    d-3                     30                                                    d-4      Steels         17                                                    d-5                     8                                                     ______________________________________                                    

                                      Table 10                                    __________________________________________________________________________                                  Deoxidation                                                                   Product                                                                              Composition of Deoxi-                    No. Samples                                                                            Chemical Composition (%)                                                                           (g/ton dation Products (%)                                                                            (SiO.sub.2)+(MnO)+               C  Si Mn Cr Pb  Ca   Steel) SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   Others                                                                            (Al.sub.2 O.sub.3)                                                            (%)                     __________________________________________________________________________    D-1      0.12                                                                             0.28                                                                             0.45                                                                             0.31                                                                             0.03                                                                              --   353    60  23   5   12  88                          Steel of                                                                  D-2      0.21                                                                             0.25                                                                             0.80                                                                             0.90                                                                             0.05                                                                              --   362    58  20  14    8  92                          this In-                                                                  D-3      0.35                                                                             0.15                                                                             0.73                                                                             1.05                                                                             0.07                                                                              --   225    48  17  23   12  88                          vention                                                                   D-4      0.48                                                                             0.34                                                                             0.83                                                                             1.13                                                                             0.09                                                                              --   133    50  10  25   15  85                      D-5      0.64                                                                             0.28                                                                             0.98                                                                             0.82                                                                             0.08                                                                              --   100    48  12  25   15  85                      __________________________________________________________________________    d-1      0.11                                                                             0.24                                                                             0.43                                                                             0.31                                                                             --  0.001                                                                              348    53  2   24   21  79                          Conven-                                                                   d-2      0.20                                                                             0.23                                                                             0.77                                                                             0.91                                                                             --  0.001                                                                              311    50  0   31   19  81                          tional                                                                    d-3      0.35                                                                             0.17                                                                             0.71                                                                             1.02                                                                             --  0.002                                                                              230    54  1   29   17  83                          Steels                                                                    d-4      0.49                                                                             0.31                                                                             0.80                                                                             1.15                                                                             --  0.003                                                                              123    48  0   39   13  87                      d-5      0.63                                                                             0.28                                                                             0.97                                                                             0.79                                                                             --  0.005                                                                               89    46  0   42   12  88                      __________________________________________________________________________

EXAMPLE 4

Chromium-Molybdenum steels of the compositions given in Table 12 wereprepared.

The tempered steel specimens were subjected to cutting test under theconditions indicated also in Table 8. The determined high speed steeltool life is shown in Table 13.

                  Table 13                                                        ______________________________________                                                            Tool Life of High Speed                                   No.      Samples    Steel Tool (min)                                          ______________________________________                                        E-1                 210                                                                Steel of                                                             E-2                 68                                                                 this In-                                                             E-3                 29                                                                 vention                                                              E-4                 11                                                        ______________________________________                                        e-1                 105                                                                Conventional                                                         e-2                 25                                                                   Steel                                                              e-3                 12                                                        e-4                  4                                                        ______________________________________                                    

                                      Table 13                                    __________________________________________________________________________                                     Deoxida-                                                                      tion                                                                          Product                                                                            Composition of Deoxida-                                                                        (SiO.sub.2)+           No. Sample                                                                             Chemical Composition (%)                                                                              (g/ton                                                                             tion Products (%)                                                                              (MnO)+                          C  Si Mn Cr Mo Pb  Ca   Steel)                                                                             SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   others                                                                            (Al.sub.2 O.sub.3)                                                            (%)                    __________________________________________________________________________    E-1      0.18                                                                             0.34                                                                             0.80                                                                             0.51                                                                             0.13                                                                             0.04                                                                              --   348  53  28   7   12  88                         Steel of                                                                  E-2      0.35                                                                             0.25                                                                             0.81                                                                             1.02                                                                             0.20                                                                             0.07                                                                              --   218  49  17  20   14  86                         this In-                                                                  E-3      0.45                                                                             0.25                                                                             0.95                                                                             1.05                                                                             0.23                                                                             0.08                                                                              --   163  52  11  23   14  86                         vention                                                                   E-4      0.65                                                                             0.28                                                                             0.75                                                                             0.73                                                                             0.32                                                                             0.05                                                                              --   105  52  10  25   13  87                     __________________________________________________________________________    e-1      0.18                                                                             0.33                                                                             0.79                                                                             0.53                                                                             0.15                                                                             --  0.002                                                                              333  62  0   20   18  82                         Coven-                                                                    e-2      0.33                                                                             0.23                                                                             0.83                                                                             1.00                                                                             0.21                                                                             --  0.004                                                                              220  53  0   32   15  85                         tional                                                                    e-3      0.43                                                                             0.27                                                                             0.93                                                                             1.02                                                                             0.25                                                                             --  0.004                                                                              157  51  0   33   16  84                         Steels                                                                    e-4      0.63                                                                             0.26                                                                             0.73                                                                             0.71                                                                             0.32                                                                             --  0.005                                                                               93  50  0   36   14  86                     __________________________________________________________________________

EXAMPLE 5

Manganese steel of the compositions given in Table 15 were prepared.

The tempered steel specimens were subjected to cutting test under theconditions indicated also in Table 8.

The high speed steel tool life observed is shown in Table 15.

                  Table 15                                                        ______________________________________                                                                Tool Life of High Speed                               No.      Samples        Steel Tool (min)                                      ______________________________________                                        F-1      Steel of       157                                                   F-2      this In-       85                                                    F-3      vention        35                                                    ______________________________________                                        f-1                     92                                                             Conventional                                                         f-2                     42                                                             Steels                                                               f-3                     15                                                    ______________________________________                                    

                                      Table 15                                    __________________________________________________________________________                                  Deoxidation                                                                   Product                                                                              Composition of Deoxi-                                                                          (SiO.sub.2)+(MnO)+      No. Samples                                                                            Chemical Compositions (%)                                                                          (g/ton dation Product (%)                                                                             (Al.sub.2 O.sub.3)               C   Si  Mn  Pb  Ca   steel) SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   Others                      __________________________________________________________________________    F-1 Steel of                                                                           0.17                                                                              0.16                                                                              1.23                                                                              0.07                                                                              --   338    57  22  6    15  85                          this In-                                                                  F-2      0.30                                                                              0.30                                                                              1.75                                                                              0.04                                                                              --   227    47  16  25   12  88                          vention                                                                   F-3      0.48                                                                              0.34                                                                              1.87                                                                              0.06                                                                              --   120    54  10  25   11  89                      __________________________________________________________________________    f-1      0.16                                                                              0.15                                                                              1.25                                                                              --  0.002                                                                              327    63  0   18   19  81                          Conven-                                                                   f-2      0.29                                                                              0.33                                                                              1.72                                                                              --  0.003                                                                              213    51  0   33   16  84                          tional                                                                    f-3      0.49                                                                              0.31                                                                              1.85                                                                              --  0.005                                                                              111    44  0   41   15  85                          Steels                                                                    __________________________________________________________________________

EXAMPLE 6

Manganese-Chromium steels of the compositions given in Table 17 wereprepared. The steel specimens were annealed and subjected to cuttingtest under the conditions indicated also in Table 8.

The high speed steel tool life obtained is shown in Table 18.

                  Table 18                                                        ______________________________________                                                                Tool life of high speed                               No.      Samples        steel tool (min)                                      ______________________________________                                        G-1      Steel of this  150                                                   G-2      Invention      30                                                    ______________________________________                                        g-1      Conventional   65                                                    g-2      Steels         11                                                    ______________________________________                                    

                                      Table 17                                    __________________________________________________________________________                                  Deoxidation                                                                   Product                                                                              Composition of Deoxi-                                                                          (SiO.sub.2)+(MnO)+               Chemical Compositions (%)                                                                          (g/ton dation Products (%)                                                                            (Al.sub.2 O.sub.3)                                                            (%)                     No. Samples                                                                            C  Si Mn Cr Pb  Ca   steel) SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   Others                      __________________________________________________________________________    G-1 Steel of                                                                           0.18                                                                             0.28                                                                             1.23                                                                             0.35                                                                             0.05                                                                              --   253    54  21  10   15  85                          this In-                                                                  G-2 vention                                                                            0.46                                                                             0.20                                                                             1.60                                                                             0.65                                                                             0.04                                                                              --   123    50  13  23   14  86                      __________________________________________________________________________    g-1 Conven-                                                                            0.20                                                                             0.26                                                                             1.20                                                                             0.33                                                                             --  0.002                                                                              255    52   0  31   17  83                          tional                                                                    g-2 Steels                                                                             0.48                                                                             0.21                                                                             1.62                                                                             0.65                                                                             --  0.004                                                                              138    51   0  35   14  86                      __________________________________________________________________________

EXAMPLE 7

Molybdenum steels of the compositions given in Table 19 were prepared.

The annealed steel specimens were subjected to cutting test under thecondition in Table 8.

The results are shown in Table 20.

                  Table 20                                                        ______________________________________                                                             Tool Life of High Speed                                  No.     Samples      Steel Tool    (min)                                      ______________________________________                                        H-1                  520                                                              Steel of this                                                         H-2                  350                                                              Invention                                                             H-3                  95                                                       H-4                  40                                                       ______________________________________                                        h-1                  280                                                              Conventional                                                          h-2                  160                                                              Steels                                                                h-3                  40                                                       h-4                  18                                                       ______________________________________                                    

                                      Table 19                                    __________________________________________________________________________                                  Deoxidation                                                                   Product                                                                              Composition of Deoxi-                                                                          (SiO.sub.2)+(MnO)+      No. Samples                                                                            Chemical Compositions (%)                                                                          (g/ton dation Product (%)                                                                             (Al.sub.2 O.sub.3)                                                            (%)                              C  Si Mn Mo Pb  Ca   steel) SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   Others                      __________________________________________________________________________    H-1      0.09                                                                             0.25                                                                             0.97                                                                             0.20                                                                             0.04                                                                              --   439    57  23   5   15  85                          Steel of                                                                  H-2      0.18                                                                             0.33                                                                             0.47                                                                             0.55                                                                             0.05                                                                              --   373    53  20  14   13  87                          this In-                                                                  H-3      0.35                                                                             0.21                                                                             0.81                                                                             0.21                                                                             0.08                                                                              --   225    47  16  23   14  86                          vention                                                                   H-4      0.50                                                                             0.28                                                                             0.94                                                                             0.25                                                                             0.04                                                                              --   153    54  11  24   11  89                      __________________________________________________________________________    h-1      0.08                                                                             0.23                                                                             0.96                                                                             0.21                                                                             --  0.001                                                                              430    52  1   29   18  82                          Conven-                                                                   h-2      0.17                                                                             0.33                                                                             0.48                                                                             0.57                                                                             --  0.002                                                                              353    51  0   32   17  82                          tional                                                                    h-3      0.36                                                                             0.18                                                                             0.78                                                                             0.20                                                                             --  0.003                                                                              215    43  0   42   15  85                          Steels                                                                    h-4      0.55                                                                             0.25                                                                             0.98                                                                             0.24                                                                             --  0.003                                                                              147    40  0   49   13  87                      __________________________________________________________________________

EXAMPLE 8

Nickel-Chromium steels of the compositions given in Table 21 wereprepared.

The annealed steel specimens were subjected to cutting test under theconditions in Table 8.

The results are shown in Table 22.

                  Table 22                                                        ______________________________________                                                             Tool Life of High Speed                                  No.     Samples      Steel Tools    (min)                                     ______________________________________                                        I-1                  81                                                               Steel of this                                                         I-2                  24                                                               Invention                                                             I-3                  30                                                       ______________________________________                                        i-1                  45                                                               Conventional                                                          i-2                  6                                                                Steels                                                                i-3                  18                                                       ______________________________________                                    

                                      Table 21                                    __________________________________________________________________________                                      Deoxida-                                                                      tion                                                                          Product                                                                            Composition of Deoxi-                                                                          (SiO.sub.2)+                   Chemical Compositions (%)                                                                              (g/ton                                                                             dation Product (%)                                                                             (MnO)+                No. Samples                                                                            c   Si Mn Ni Cr Pb  Ca   steel)                                                                             SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   Others                                                                            (Al.sub.2                                                                     O.sub.3)              __________________________________________________________________________                                                            (%)                   I-1 Steel of                                                                           0.12                                                                              0.33                                                                             0.50                                                                             2.35                                                                             0.25                                                                             0.05                                                                              --   440  53  27   5   15  85                    I-2 this In-                                                                           0.33                                                                              0.25                                                                             0.50                                                                             3.42                                                                             0.95                                                                             0.08                                                                              --   218  55  20  14   11  89                    I-3 vention                                                                            0.38                                                                              0.17                                                                             0.80                                                                             1.10                                                                             0.80                                                                             0.04                                                                              --   142  55  11  24   10  90                    __________________________________________________________________________    i-1 Conven-                                                                            0.10                                                                              0.31                                                                             0.48                                                                             2.40                                                                             0.23                                                                             --  0.002                                                                              426  63  0   24   13  87                    i-2 tional                                                                             0.35                                                                              0.25                                                                             0.49                                                                             3.45                                                                             1.00                                                                             --  0.003                                                                              217  50  1   37   12  88                    i-3 Steels                                                                             0.38                                                                              0.18                                                                             0.81                                                                             1.02                                                                             0.83                                                                             --  0.004                                                                              123  50  0   37   13  87                    __________________________________________________________________________

EXAMPLE 9

Nickel-Chromium-Molybdenum steels of the compositions indicated in Table23 were prepared.

After annealing, the steel specimens were subjected to cutting testunder the conditions given in Table 8.

Table 24 shows the results of the test.

                  Table 24                                                        ______________________________________                                                             Tool Life of High Speed                                  No.     Samples      Steel Tool    (min)                                      ______________________________________                                        J-1                  40                                                       J-2                  15                                                       J-3     Steel of this                                                                              10                                                       J-4                  12                                                               Invention                                                             J-5                  10                                                       J-6                  6                                                        ______________________________________                                        j-1                  20                                                       j-2                  7                                                        j-3     Conventional 3                                                        j-4                  4                                                                Steels                                                                j-5                  5                                                        j-6                  2                                                        ______________________________________                                    

                                      Table 23                                    __________________________________________________________________________                                      Deoxida-                                                                      tion                                                 Chemical Compositions (%)                                                                              Product                                                                            Composition of Deoxida-                                                                        (SiO.sub.2)+                                            (g/ton                                                                             tion Products (%)                                                                              (MnO)+                No. Samples                                                                            C  Si Mn Ni Cr Mo Pb Ca  steel)                                                                             SiO.sub.2                                                                         MnO Al.sub.2 O.sub.3                                                                   Others                                                                            (Al.sub.2                                                                     O.sub.3)              __________________________________________________________________________                                                            (%)                   J-1      0.08                                                                             0.28                                                                             0.45                                                                             3.25                                                                             1.23                                                                             0.10                                                                             0.05                                                                             --  447  50  28   7   15  85                    J-2 Steel of                                                                           0.20                                                                             0.22                                                                             1.20                                                                             3.00                                                                             1.60                                                                             0.50                                                                             0.03                                                                             --  341  53  24  12   11  89                    J-3 this In-                                                                           0.30                                                                             0.25                                                                             0.55                                                                             3.00                                                                             3.45                                                                             0.60                                                                             0.05                                                                             --  257  56  21  10   13  87                    J-4 vention                                                                            0.40                                                                             0.25                                                                             0.85                                                                             0.55                                                                             0.50                                                                             0.19                                                                             0.07                                                                             --  181  47  16  22   15  85                    J-5      0.48                                                                             0.17                                                                             0.90                                                                             1.90                                                                             0.85                                                                             0.70                                                                             0.08                                                                             --  156  50  17  23   10  90                    J-6      0.64                                                                             0.28                                                                             1.00                                                                             0.60                                                                             0.55                                                                             0.18                                                                             0.04                                                                             --  103  55  12  25   8   92                    __________________________________________________________________________    j-1      0.09                                                                             0.30                                                                             0.44                                                                             3.23                                                                             1.25                                                                             0.10                                                                             -- 0.001                                                                             453  49  0   44   7   93                    j-2 Conven-                                                                            0.22                                                                             0.20                                                                             1.18                                                                             2.95                                                                             1.55                                                                             0.15                                                                             -- 0.002                                                                             306  49  1   44   6   94                    j-3 tional                                                                             0.31                                                                             0.25                                                                             0.53                                                                             3.02                                                                             3.40                                                                             0.60                                                                             -- 0.002                                                                             221  51  0   38   11  89                    j-4 Steels                                                                             0.42                                                                             0.25                                                                             0.80                                                                             0.60                                                                             0.52                                                                             0.17                                                                             -- 0.003                                                                             200  48  0   42   10  90                    j-5      0.50                                                                             0.17                                                                             0.91                                                                             1.80                                                                             0.88                                                                             0.75                                                                             -- 0.003                                                                             161  40  0   54   6   94                    j-6      0.65                                                                             0.29                                                                             1.05                                                                             0.62                                                                             0.51                                                                             0.20                                                                             -- 0.003                                                                              81  43  0   50   7   93                    __________________________________________________________________________

In short, the distribution of Pb in conventional free cutting steels ofthe Ca-Pb type was not good; on the contrary the distribution of Pb insteel of this invention has been extremely improved by employing aspecific deoxidizing agent which is effective for reduction of themelting point of the deoxidation product, adjusting the sum of contentsof SiO₂, MnO and Al₂ O₃ to at least 85% of the whole oxides of thedeoxidation product, and thereafter adding Pb to the steel thereby todistribute Pb uniformly in the form of non-metallic inclusionscomposited with sulfides and finely divided Pb particles.

The free cutting steel of the present invention can exhibit a highlyimproved tool life when it is cut by a high speed steel tool. Thus, thefree cutting steel of this invention has very excellent machinability.

We claim:
 1. A free cutting steel for machine structural use having anexcellent machinability for cutting with a high speed steel cuttingtool, which consisting essentially of carbon 0.08 to 0.65%, silicon 0.10to 0.35%, manganese 0.30 to 1.90%, nickel 0 to 4.50%, chromium 0 to3.50%, molybdenum 0 to 0.70%, lead 0.02 to 0.10%, and the balance ironand inherent impurities, and further contains oxide inclusion within arange from 100 to 450 grams per ton of the steel, lead within the rangeof 0.02 to 0.10 percent by weight of the said oxide inclusion beinguniformly dispersed composite inclusion consisting essentially of SiO₂40 to 60% by weight, MnO 10 to 30%, Al₂ O₃ 5 to 25% and MO up to 15%,wherein MO is an oxide of a metal selected from the group consisting ofcalcium, magnesium and iron, and the sum of SiO₂, MnO and Al₂ O₃ amountsto at least 85% of the oxide inclusion, and substantially all of thelead being uniformly distributed in the steel as finely dividedparticles in the form of composite inclusions comprising sulfides andfinely divided lead particles.
 2. A free cutting steel according toclaim 1, wherein said steel for machine structural use is a plain carbonsteel consisting of 0.08 to 0.60% of carbon, 0.10 to 0.30% of silicon,0.30 to 1.65% of manganese and the remainder essentially iron andinherent impurities.
 3. A free cutting steel according to claim 1,wherein the steel for machine structural use is a plain carbon steelcomprising 0.08 to 0.61% carbon, 0.15 to 0.35% silicon, 0.30 to 0.90%manganese and the remainder essentially iron and inherent impurities. 4.A free cutting steel according to claim 1, wherein the steel for machinestructural use is a plain carbon steel comprising 0.40 to 0.55% carbon,0.15 to 0.35% silicon, 0.60 to 1.00% manganese and the remainderessentially iron and inherent impurities.
 5. A free cutting steelaccording to claim 1, wherein the steel for machine structural uses is achromium steel comprising 0.12 to 0.64% carbon, 0.20 to 0.35% silicon,0.30 to 1.00% manganese, 0.30 to 1.15% chromium and the remainderessentially iron and inherent impurities.
 6. A free cutting steelaccording to claim 1, wherein the steel for machine structural use is achromium steel comprising 0.13 to 0.48% carbon, 0.15 to 0.35% silicon,0.60 to 0.85% manganese, 0.90 to 1.20% chromium and the remainderessentially iron and inherent impurities.
 7. A free cutting steelaccording to claim 1, wherein the steel for machine structural use is achromium-molybdenum steel comprising 0.18 to 0.65% carbon, 0.15 to 0.35%silicon, 0.30 to 1.00% manganese, 0.70 to 1.20% chromium, 0.08 to 0.35%molybdenum and the remainder essentially iron and inherent impurities.8. A free cutting steel according to claim 1, wherein the steel formachine structural use is a manganese steel comprising 0.17 to 0.48%carbon, 0.15 to 0.35% silicon, 1.20 to 1.90% manganese and the remainderessentially iron and inherent impurities.
 9. A free cutting steelaccording to claim 1, wherein the steel for machine structural use is amolybdenum steel comprising 0.09 to 0.50% carbon, 0.20 to 0.35% silicon,0.65 to 1.00% manganese, 0.15 to 0.60% molybdenum and the remainderessentially iron and inherent impurities.
 10. A free cutting steelaccording to claim 1, wherein the steel for machine structural use is amanganese-chromium steel comprising 0.17 to 0.46% carbon, 0.15 to 0.35%silicon, 1.20 to 1.65% manganese, 0.35 to 0.70% chromium and theremainder essentially iron and inherent impurities.
 11. A free cuttingsteel according to claim 1, wherein the steel for machine structural useis a chromium-molybdenun steel comprising 0.18 to 0.23% carbon, 0.15 to0.35% silicon, 0.60 to 0.85% manganese, 0.90 to 1.20% chromium, 0.15 to0.30% molybdenum and the remainder essentially iron and inherentimpurities.
 12. A free cutting steel according to claim 1, wherein thesteel for machine structural use is a nickel-chromium steel comprising0.12 to 0.40% carbon, 0.15 to 0.35% silicon, 0.35 to 0.80% manganese,1.00 to 3.50% nickel, 0.20 to 1.00% chromium and the remainderessentially iron and inherent impurities.
 13. A free cutting steelaccording to claim 1, wherein the steel for machine structural use is anickel-chromium-molybdenum steel comprising 0.08 to 0.64% carbon, 0.20to 0.35% silicon, 0.45 to 1.00% manganese, 0.40 to 3.50% nickel, 0.30 to1.40% chromium, 0.08 to 0.40% molybdenum and the remainder essentiallyiron and inherent impurities.
 14. A free cutting steel according toclaim 1, wherein the steel for machine structural use is anickel-chromium-molybdenum steel comprising 0.12 to 0.50% carbon, 0.15to 0.35% silicon, 0.35 to 1.20% manganese, 0.40 to 4.50% nickel, 0.40 to3.50% chromium, 0.15 to 0.70% molybdenum and the remainder essentiallyiron and inherent impurities.